1. Field of the Invention
The invention relates to machines, manufactures, and processes and in particular to systems and methods for sensing jitter from speckle patterns.
2. Discussion of the Related Art
Jitter is a movement such as an irregular random movement. Jitter can be a detrimental influence in measurement systems. For example, jitter in systems including a motion sensor or imaging device such as a camera is undesirable as it tends to blur the measurement or image. Jitter sensitive systems may include a means for detecting and measuring jitter in order to, among other things, evaluate the effectiveness of jitter mitigation, mitigate the impact of blur, and discard blurred measurements.
Pointing jitter is frequently a performance constraint on space borne imaging sensors. In these systems, pointing jitter is typically measured using spacecraft attitude control system sensors or photodetectors. In particular, accelerometers used in spacecraft attitude control can be used to measure jitter. The jitter measured in this case is the jitter of the spacecraft.
FIG. 1 shows a schematic of a typical camera support 100. Here, a spacecraft or another structure 112 supports the camera 102. To the degree the spacecraft and camera are not rigid bodies with a rigid interconnection, one is free to move with respect to the other according to a complex transfer function 108.
An attitude control sensor 116 measures jitter of the spacecraft. Notably, the sensor does not measure jitter of the camera 102 or jitter of the camera boresight 104. However, if a transfer function 108 relating spacecraft jitter to camera boresight jitter is known, the camera boresight jitter can be predicted based on spacecraft jitter.
FIG. 2 shows a pixel scale indirect imaging system using autocollimation 200. Measuring system components include a light source 202, a mirror 204, and a quadrant detector 206 consisting of a 2×2 grid of photodetectors 208, 210, 212, 214.
The mirror 204 is attached to a device of interest such as a spacecraft or camera 207. During operation, the mirror reflects light 205 received 203 from the light source and a spot 221 at the center of the quadrant detector 223 is illuminated by the reflected light beam.
Where the mirror and one of the light source and the detector are located in different reference planes, for example on earth and in space, relative motion of the detector causes more or less light to shine on particular cells. The relative intensity of the four quadrants is used to determine the change in angle relative to the reference mirror and to indicate jitter.
As can be seen, the autocollimation sensor does not measure jitter of the camera boresight 104. However, if a transfer function 108 relating spacecraft jitter to camera boresight jitter is known, the camera boresight jitter can be predicted based on spacecraft jitter.
FIG. 3 shows a pixel scale direct imaging system 300. Direct imaging refers to imaging on the camera whose boresight jitter is to be measured. A point light source 310 and a camera 305 are located in different reference frames. The point light source is directed toward the center 304 of the camera's pixel array 302. A light beam 311 from the light source illuminates a spot 306 at the center of the array. Movement of the camera relative to the light source moves the light spot to a different location on the array. Pixel readings of light intensity indicate this movement and provide a measure of the extent and direction of camera boresight jitter.
As persons of ordinary skill in the art will appreciate, direct imaging of a projected point source is a pixel scale measuring technique. In addition, accuracy of this method is limited to the centroid noise of the camera. In summary, these prior art techniques for measuring pointing jitter suffer from one or more disadvantages including indirect measurement of pointing jitter, accuracy limitations due to pixel scale resolution, and accuracy limitations due to centroid noise of the camera.